1. Field of the Invention
The present invention relates to a hydraulic power steering device for use in motor vehicles or the like, and particularly, it relates to a variable gear ratio hydraulic power steering device with a variable steering gear ratio.
2. Discussion of the Related Art
Heretofore, as hydraulic power steering devices, there has been known one for example which is provided with a flow volume control valve for controlling the flow volume of operating fluid supplied from an oil pump to a power cylinder and which is constructed to control the flow volume control valve electronically, as disclosed for example in Japanese exampled, published patent No. 2-56274. In the power steering device, a vehicle speed, a handle steering angle, a handle angular velocity and so on are taken as control inputs, and the discharge flow volume of the oil pump is regulated by controlling the valve opening degree of the flow volume control valve based on these control inputs. Thus, steering assist force to be generated is properly controlled by regulating the discharge flow volume of the oil pump, so that the handle steering feeling felt by the driver and the attainment of energy saving are aimed at being compatible with each other.
Also as hydraulic power steering devices, there has been known one for example which is provided with a variable ratio transmission mechanism for varying the rotation transmission ratio between a steering shaft coupled to a steering handle and a pinion shaft coupled to a pinion gear in engagement with a rack gear and which is constructed to make the steering gear ratio variable, as disclosed for example in Japanese unexampled, published patent application No. 10-324263. With the variable ratio transmission mechanism, it can be realized to set the steering gear ratio to a quick side in a low speed range for reduction of the handle steering amount by the driver and to set the steering gear ratio to a slow side in a high speed range for enhancement of the traveling stability.
Further, as hydraulic power steering devices, there has been known a variable gear ratio power steering device for example which is constituted by combining the aforementioned flow volume control mechanism for controlling the discharge flow volume of the oil pump with the aforementioned variable ratio transmission mechanism for making the rotation transmission ratio variable.
However, the variable gear ratio hydraulic power steering device in the prior art involves the following problems. That is, since the operation of the variable ratio transmission mechanism causes the steering gear ratio to fluctuate, variation occurs in the flow volume of operating fluid required by a power cylinder. When the discharge flow volume of the oil pump runs short less than the required flow volume, it tends to occur that the steering assist force to be generated runs short, whereby a large force is needed to manipulate the steering handle.
On the contrary, it is possible for example to set the discharge flow volume for the state that the steering gear ratio is set by the variable ratio transmission mechanism to the end of the quick side. However, in the case so set, it tends to occur that the substantial part of the discharge flow volume from the oil pump is consumed uselessly with the energy saving degraded in effect.